We are using Lightstreamer Moderato Server version 3.5 build 1422. I don't have a way to reproduce this issue either. It just happened intermittently. But I found hundreds of such <Sync error> log statements in the daily LS server log file.

Here are the relevant log entries related to the particular user.

2009-08-10 08:51:29,250 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/create_session.js?LS_phase=1702&LS_domain=currense e.com&LS_polling=true&LS_polling_millis=0&LS_idle_ millis=30000&LS_client_version=4.3&LS_adapter=FORE XPERT&LS_user=8&LS_password=[...]& from 82.81.254.65:4605>
2009-08-10 08:51:29,274 INFO [LightstreamerLogger.requests] - <Starting new session: S4040fce00115c0a5T5129274 from 82.81.254.65:4605>
2009-08-10 08:51:29,942 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/control.html?LS_session=S4040fce00115c0a5T5129274& LS_window=1&LS_win_phase=4&LS_op=add&LS_req_phase= 93&LS_mode1=MERGE&LS_id1=FXCMPriceUpdates%20thomso nNewsEvent%20publish-8%20consume-8%20Discussions%20jms.thomson.reports%20PlatformAn nouncements%20economicEventTopic%20publish-59%20publish-321%20publish-457%20publish-227%20publish-1%20publish-296%20publish-200%20publish-429%20publish-10%20publish-373%20publish-872%20publish-160%20publish-135%20publish-416%20publish-598%20publish-3%20publish-48%20publish-247%20publish-214%20publish-313%20publish-427%20publish-21%20publish-418%20publish-195%20publish-33%20publish-507%20publish-228%20publish-1356%20publish-92%20publish-194%20publish-264%20publish-1384%20publish-162%20publish-119%20publish-35%20publish-58%20publish-417%20publish-419%20publish-325%20publish-86%20publish-276%20publish-185%20publish-80%20publish-204%20publish-275%20publish-38%20publish-47%20publish-234%20publish-105%20publish-508%20publish-1073%20publish-331%20publish-145%20publish-350%20publish-362%20publish-611%20publish-215%20publish-232%20publish-613%20publish-188%20publish-211%20publish-473%20publish-30%20publish-502%20publish-1088%20publish-565%20publish-236%20publish-170%20publish-346%20publish-231%20publish-361%20publish-193%20publish-168%20publish-240%20publish-229%20publish-312%20publish-37%20publish-212%20publish-394%20publish-123%20publish-609%20publish-79%20publish-399%20publish-2%20publish-253%20publish-316%20publish-237%20publish-15%20publish-332%20publish-39%20publish-326%20publish-300%20publish-73%20publish-72%20publish-448%20publish-420%20publish-307%20publish-266%20publish-265%20publish-52%20publish-217%20publish-343%20publish-256%20publish-510%20publish-561&LS_schema1=message&LS_unique=1 from 82.81.254.65:4605>
2009-08-10 08:51:29,942 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4605>
2009-08-10 08:51:30,214 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1703&LS_domain=currensee.com& from 82.81.254.65:4609>
2009-08-10 08:51:30,214 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:4609>
2009-08-10 08:52:59,383 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1704&LS_domain=currensee.com& from 82.81.254.65:4715>
2009-08-10 08:52:59,383 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:4715>
2009-08-10 08:54:19,412 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/control.html?LS_session=S4040fce00115c0a5T5129274& LS_window=2&LS_win_phase=4&LS_op=add&LS_req_phase= 94&LS_mode1=MERGE&LS_id1=feed-1&LS_schema1=message&LS_unique=2 from 82.81.254.65:4803>
2009-08-10 08:54:19,412 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Serving multiple request: /lightstreamer/control.html?{8 subrequests} from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 1: LS_session=S4040fce00115c0a5T5129274&LS_window=3&L S_win_phase=4&LS_op=add&LS_req_phase=95&LS_mode1=M ERGE&LS_id1=feed-2&LS_schema1=message&LS_unique=3 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 2: LS_session=S4040fce00115c0a5T5129274&LS_window=4&L S_win_phase=4&LS_op=add&LS_req_phase=96&LS_mode1=M ERGE&LS_id1=feed-3&LS_schema1=message&LS_unique=4 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 3: LS_session=S4040fce00115c0a5T5129274&LS_window=5&L S_win_phase=4&LS_op=add&LS_req_phase=97&LS_mode1=M ERGE&LS_id1=feed-4&LS_schema1=message&LS_unique=5 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 4: LS_session=S4040fce00115c0a5T5129274&LS_window=6&L S_win_phase=4&LS_op=add&LS_req_phase=98&LS_mode1=M ERGE&LS_id1=feed-6&LS_schema1=message&LS_unique=6 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 5: LS_session=S4040fce00115c0a5T5129274&LS_window=7&L S_win_phase=4&LS_op=add&LS_req_phase=99&LS_mode1=M ERGE&LS_id1=feed-8&LS_schema1=message&LS_unique=7 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 6: LS_session=S4040fce00115c0a5T5129274&LS_window=8&L S_win_phase=4&LS_op=add&LS_req_phase=100&LS_mode1= MERGE&LS_id1=feed-9&LS_schema1=message&LS_unique=8 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 7: LS_session=S4040fce00115c0a5T5129274&LS_window=9&L S_win_phase=4&LS_op=add&LS_req_phase=101&LS_mode1= MERGE&LS_id1=feed-15&LS_schema1=message&LS_unique=9 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Subrequest 8: LS_session=S4040fce00115c0a5T5129274&LS_window=10& LS_win_phase=4&LS_op=add&LS_req_phase=102&LS_mode1 =MERGE&LS_id1=feed-16&LS_schema1=message&LS_unique=10 from 82.81.254.65:4803>
2009-08-10 08:54:20,680 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:21,112 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:21,540 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:22,036 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:22,484 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:22,896 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:23,348 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:23,908 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:26,992 INFO [LightstreamerLogger.requests] - <Serving multiple request: /lightstreamer/control.html?{2 subrequests} from 82.81.254.65:4803>
2009-08-10 08:54:26,992 INFO [LightstreamerLogger.requests] - <Subrequest 1: LS_session=S4040fce00115c0a5T5129274&LS_window=9&L S_win_phase=4&LS_op=add&LS_req_phase=101&LS_mode1= MERGE&LS_id1=feed-15&LS_schema1=message&LS_unique=11 from 82.81.254.65:4803>
2009-08-10 08:54:26,992 INFO [LightstreamerLogger.requests] - <Subrequest 2: LS_session=S4040fce00115c0a5T5129274&LS_window=10& LS_win_phase=4&LS_op=add&LS_req_phase=102&LS_mode1 =MERGE&LS_id1=feed-16&LS_schema1=message&LS_unique=12 from 82.81.254.65:4803>
2009-08-10 08:54:26,992 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:27,396 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:54:38,185 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1705&LS_domain=currensee.com& from 82.81.254.65:4803>
2009-08-10 08:54:38,185 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:4803>
2009-08-10 08:56:07,874 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1706&LS_domain=currensee.com& from 82.81.254.65:4875>
2009-08-10 08:56:07,874 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:4875>
2009-08-10 08:57:33,919 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1707&LS_domain=currensee.com& from 82.81.254.65:4942>
2009-08-10 08:57:33,919 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:4942>

[...] Skipped many repetition of STREAMING_IN_PROGRESS log entries

2009-08-10 10:33:30,707 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1770&LS_domain=currensee.com& from 82.81.254.65:2315>
2009-08-10 10:33:30,707 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:2315>
2009-08-10 10:35:06,508 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1771&LS_domain=currensee.com& from 82.81.254.65:2337>
2009-08-10 10:35:06,508 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:2337>
2009-08-10 10:36:43,742 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1772&LS_domain=currensee.com& from 82.81.254.65:2353>
2009-08-10 10:36:43,742 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:2353>
2009-08-10 10:38:16,423 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1773&LS_domain=currensee.com& from 82.81.254.65:2377>
2009-08-10 10:38:16,423 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:2377>
2009-08-10 10:39:50,264 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/STREAMING_IN_PROGRESS?LS_session=S4040fce00115c0a5 T5129274&LS_phase=1774&LS_domain=currensee.com& from 82.81.254.65:2397>
2009-08-10 10:39:50,264 INFO [LightstreamerLogger.requests] - <Attaching session: S4040fce00115c0a5T5129274 from 82.81.254.65:2397>
2009-08-10 10:39:50,600 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/create_session.js?LS_phase=1775&LS_domain=currense e.com&LS_polling=true&LS_polling_millis=1109&LS_id le_millis=30000&LS_client_version=4.3&LS_adapter=F OREXPERT&LS_user=8&LS_password=[...]&LS_old_session=S4040fce00115c0a5T5129274& from 82.81.254.65:2399>
2009-08-10 10:39:50,716 INFO [LightstreamerLogger.requests] - <Starting new session: Sb50bef2560eac1cbT3950716 from 82.81.254.65:2399>
2009-08-10 10:39:51,084 INFO [LightstreamerLogger.requests] - <Serving request: /lightstreamer/control.html?LS_session=S4040fce00115c0a5T5129274& LS_window=11&LS_win_phase=5&LS_op=add&LS_req_phase =103&LS_mode1=MERGE&LS_id1=FXCMPriceUpdates%20thom sonNewsEvent%20publish-8%20consume-8%20Discussions%20jms.thomson.reports%20PlatformAn nouncements%20economicEventTopic%20publish-59%20publish-321%20publish-457%20publish-227%20publish-1%20publish-296%20publish-200%20publish-429%20publish-10%20publish-373%20publish-872%20publish-160%20publish-135%20publish-416%20publish-598%20publish-3%20publish-48%20publish-247%20publish-214%20publish-313%20publish-427%20publish-21%20publish-418%20publish-195%20publish-33%20publish-507%20publish-228%20publish-1356%20publish-92%20publish-194%20publish-264%20publish-1384%20publish-162%20publish-119%20publish-35%20publish-58%20publish-417%20publish-419%20publish-325%20publish-86%20publish-276%20publish-185%20publish-80%20publish-204%20publish-275%20publish-38%20publish-47%20publish-234%20publish-105%20publish-508%20publish-1073%20publish-331%20publish-145%20publish-350%20publish-362%20publish-611%20publish-215%20publish-232%20publish-613%20publish-188%20publish-211%20publish-473%20publish-30%20publish-502%20publish-1088%20publish-565%20publish-236%20publish-170%20publish-346%20publish-231%20publish-361%20publish-193%20publish-168%20publish-240%20publish-229%20publish-312%20publish-37%20publish-212%20publish-394%20publish-123%20publish-609%20publish-79%20publish-399%20publish-2%20publish-253%20publish-316%20publish-237%20publish-15%20publish-332%20publish-39%20publish-326%20publish-300%20publish-73%20publish-72%20publish-448%20publish-420%20publish-307%20publish-266%20publish-265%20publish-52%20publish-217%20publish-343%20publish-256%20publish-510%20publish-561&LS_schema1=message&LS_unique=13 from 82.81.254.65:2401>
2009-08-10 10:39:51,084 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2401>
2009-08-10 10:39:51,084 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2401>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Serving multiple request: /lightstreamer/control.html?{9 subrequests} from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 1: LS_session=S4040fce00115c0a5T5129274&LS_window=12& LS_win_phase=5&LS_op=add&LS_req_phase=104&LS_mode1 =MERGE&LS_id1=feed-1&LS_schema1=message&LS_unique=14 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 2: LS_session=S4040fce00115c0a5T5129274&LS_window=13& LS_win_phase=5&LS_op=add&LS_req_phase=105&LS_mode1 =MERGE&LS_id1=feed-2&LS_schema1=message&LS_unique=15 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 3: LS_session=S4040fce00115c0a5T5129274&LS_window=14& LS_win_phase=5&LS_op=add&LS_req_phase=106&LS_mode1 =MERGE&LS_id1=feed-3&LS_schema1=message&LS_unique=16 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 4: LS_session=S4040fce00115c0a5T5129274&LS_window=15& LS_win_phase=5&LS_op=add&LS_req_phase=107&LS_mode1 =MERGE&LS_id1=feed-4&LS_schema1=message&LS_unique=17 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 5: LS_session=S4040fce00115c0a5T5129274&LS_window=16& LS_win_phase=5&LS_op=add&LS_req_phase=108&LS_mode1 =MERGE&LS_id1=feed-6&LS_schema1=message&LS_unique=18 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 6: LS_session=S4040fce00115c0a5T5129274&LS_window=17& LS_win_phase=5&LS_op=add&LS_req_phase=109&LS_mode1 =MERGE&LS_id1=feed-8&LS_schema1=message&LS_unique=19 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 7: LS_session=S4040fce00115c0a5T5129274&LS_window=18& LS_win_phase=5&LS_op=add&LS_req_phase=110&LS_mode1 =MERGE&LS_id1=feed-9&LS_schema1=message&LS_unique=20 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 8: LS_session=S4040fce00115c0a5T5129274&LS_window=19& LS_win_phase=5&LS_op=add&LS_req_phase=111&LS_mode1 =MERGE&LS_id1=feed-15&LS_schema1=message&LS_unique=21 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Subrequest 9: LS_session=S4040fce00115c0a5T5129274&LS_window=20& LS_win_phase=5&LS_op=add&LS_req_phase=112&LS_mode1 =MERGE&LS_id1=feed-16&LS_schema1=message&LS_unique=22 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,204 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Controlling session: S4040fce00115c0a5T5129274 from 82.81.254.65:2403>
2009-08-10 10:39:51,208 INFO [LightstreamerLogger.requests] - <Sync error (transient): Can't find session from 82.81.254.65:2403>